As a conventional art, a packing device described in, e.g., PTL 1 is known. FIG. 17 is an exploded perspective view showing the packing device in the conventional art. As shown in the drawing, the packing device in the conventional art includes a quadrangular box 200 in which a packed object 100 is accommodated, and cushioning materials 300 formed by folding cardboard such as corrugated cardboard. The cushioning materials 300 are accommodated in the box 200 together with the packed object 100, and are disposed in gaps between the box 200 and the packed object 100. Flaps 400 are formed integrally on four sides of an opening portion of each of a top surface and a bottom surface of the box 200.